The present invention is generally directed to an implantable device for monitoring the progression or regression of a patient""s heart condition. The present invention is more particularly directed to a system and method for use in an implantable cardiac stimulation device which determines and stores ventricular repolarization interval dispersions over time. Relative changes in the ventricular repolarization interval dispersions, over time, are indicative of the progression or regression of the patient""s heart condition.
More people die of heart disease than any other single cause. Common forms of heart disease include congestive heart failure and malignant ventricular arrhythmias.
Congestive heart failure (CHF) is a debilitating, end-stage disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body""s tissues. When the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart""s oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the hardwalls to hamper rather than assist in providing cardiac output.
In patients with heart failure and malignant ventricular arrhythmias, the heart has often remodeled such that there is increased fibrosis between myocardial cells, a lengthening of the cells, varying degrees of hypertrophy and dilation, and up and down regulation of various receptors that effect ionic balance, action potential conduction, and contraction. These variations in the myocardial substrates often result in conduction abnormalities that increase the risk of arrhythmia.
Most cardiac patients with these heart conditions have an implanted cardiac stimulation device to administer therapy. It would therefore be most advantageous if the implanted devices were able to monitor the progression or regression of these heart conditions.
It has been observed that the ventricular repolarization duration of the hearts of these patients is spatially variable. Typically, this is measured with a standard 12 lead ECG by measuring the ventricular repolarization intervals with various lead combinations and subtracting the maximum ventricular repolarization interval from the minimum ventricular repolarization interval to derive a corresponding ventricular repolarization interval dispersion. Unfortunately, this procedure has not been practical in an implanted device because the implanted lead systems have been limited to the right side of the heart for sensing only local heart activity.
Implantable cardiac stimulation devices offering multi-chamber pacing (bi-ventricular or bi-atrial) and/or with defibrillation therapy offer an increased number of cardiac sensing electrode configurations including sensing configurations from the left side of the heart. The present invention provides a system and method for use in a multi-chamber implantable cardiac stimulation device which utilizes right and left heart sensing to advantage for measuring, over time, ventricular repolarization interval dispersion. This enables the implanted device to track the progression or regression of the patient""s heart condition to effectuate more effective therapy titration.
The present invention provides a system and method for use in an implantable cardiac stimulation device for monitoring progression or regression of a patient""s heart condition. In accordance with the present invention, the patient""s heart condition is monitored by determining a ventricular repolarization interval dispersions spaced apart over time. Relative changes in the interval dispersion changes over time are indicative of the progression or regression in the patient""s heart condition.
The ventricular repolarization interval dispersions are determined based upon the difference between a maximum ventricular repolarization interval measured with one of a plurality of electrode configurations and a minimum ventricular repolarization interval measured with another one of the plurality of electrode configurations.
The ventricular repolarization intervals may be QT intervals or alternatively, intervals beginning with a pacing stimulus and ending at the end of an immediately succeeding T-wave (Stim-T) interval. The plurality of electrode configurations may include right and left heart sensing electrode configurations to avoid localized sensing. Alternatively, two predetermined electrode configurations may be used to calculate the ventricular repolarization interval dispersion.
At each of the spaced apart times, a ventricular repolarization interval is determined for each electrode configuration. The interval dispersion is then determined from the maximum measured ventricular repolarization interval and the minimum measured ventricular repolarization interval and stored in a memory for later transmission to an external receiver by a telemetry circuit for analysis.
Pacing parameters by which the patient""s heart may be adjusted responsive to the ventricular repolarization interval dispersions. For example, if the interval dispersions show an increasing trend, an interventricular delay may be modified to change the activation sequence such that the dispersions are reduced or, if the dispersions show a decreasing trend, the pacing rate may be reduced or the AV delay may be increased to allow increased intrinsic activity of the patient""s heart.
The interval dispersions may be stored and conveyed with other parameters such as date and time of interval dispersion measurement, the heart rate, the individual ventricular repolarization intervals, and the patient""s activity level. Still further, the times for determining the ventricular repolarization interval dispersions may be set to occur when the patient is normally at rest or may be conditioned upon the patient""s heart rate or activity level being below predetermined limits. By controlling when this measurement occurs, accurate trending of the ventricular repolarization interval dispersions is facilitated.